Referring to FIG. 1, in a suspended type crane comprising a trolley mounted with a traveling apparatus, and a hoisting apparatus, the trolley 1 is generally provided with wheels 2 that roll along rails 3, and said wheels 2 being driven through a reduction apparatus 12 by a traveling motor 11 mounted on the trolley 1. An electromagnetic brake 13 and a speed detector 14 for detecting the rotating speed of the traveling motor 11 are connected with the output drive shaft of the traveling motor 11.
A hoisting apparatus 4 provided with a hoisting drive drum 41 is mounted on the trolley 1. The hoisting drive drum 41 is driven for rotation through a reduction apparatus 43 by a hoist motor 42. An electromagnetic brake 44 and a motor speed detector 45 comprising a pulse signal generator are connected with the output drive shaft of the hoist motor 42. A hoisting rope 5 is wound round the hoisting drive drum 41, and the hoisting rope 5 suspends a hoist load 6.
A travel drive control unit 20 controls the traveling motor 11 to control the traveling speed of the trolley 1. Referring to FIG. 2 showing the configuration of the travel drive control unit 20 in a block diagram, a speed reference device 21 gives a speed reference signal to a linear acceleration starter device 22. A speed regulating controller 23 provided with a proportional gain A and an integrator having a time constant .tau.1 amplifies the difference between a ramp speed reference signal N.sub.RF provided by the linear acceleration starter device 22 and a speed feedback signal N.sub.MFB provided by the speed detector 14, and provides a torque reference signal T.sub.RF. The torque reference signal T.sub.RF is given to a motor torque controller 24 which controls the torque T.sub.M of the traveling motor 11 at a first-order lag time constant .tau..sub.T to control the rotating speed of the traveling motor 11. The speed feedback signal N.sub.MFB is produced by a first-order lag element on the base of the motor. The block 25 represents the mechanical time constant .tau..sub.M of the traveling motor 11. N.sub.M is the rotating speed (p. u). The block 27 represents a kinematic model of the swing angle of the hoisting rope. The block 28 represents load torque T.sub.L (p. u) acting on the motor.
In the block 27, V.sub.R is the traveling speed (m/sec) of the trolley 1 corresponding to the rated speed of the traveling motor 11, g is the gravitational acceleration constant (m/sec.sup.2), .omega. is the angular frequency (rad/sec) of swing motion of the hoist load 6, L is the length of the hoisting rope 5, and .theta. is the swing angle (rad) of the hoisting rope 5. Therefore, .omega.=(g/L).sup.1/2.
In the block 28, m.sub.0 is the load (p. u) on the trolley 1, m.sub.1 is the weight (P. u) of the hoist load 6, and k.sub.l is a conversion factor for converting frictional torque produced by the total weight of the trolley 1 and the hoist load 6 into load torque on the driving shaft of the trolley 1.
In the travel drive control unit 20 shown in FIG. 2, when the traveling speed of the trolley 1 is controlled according to the ramp speed reference signal N.sub.RF provided by the linear acceleration starter device 22 in response to a high-speed or low-speed reference signal provided by the speed reference device 21, the hoisting rope 5 oscillates according to the acceleration and deceleration of the trolley 1. When the acceleration or deceleration of the trolley 1 increases, the swing angle of the hoisting rope 5 increases accordingly. A conventional method of stopping the oscillation of the hoisting rope has been to regulate the traveling speed of the trolley manually according to the state of sway of the hoist load during the acceleration or deceleration of the trolley.
FIG. 3 shows the respective variations of the rotating speed of the motor, the swing angle of the hoisting rope, the torque of the motor, and the load torque with variations of the speed reference signal. As is obvious from FIG. 3, the hoisting rope oscillates continuously during the acceleration and deceleration of the trolley, and the traveling speed of the trolley is unstable. In FIG. 3, the swing angle .theta. of the hoisting rope is expressed in degrees (.degree.).
Since the operator of the crane must control the trolley for acceleration or deceleration while observing the state of sway of the hoisting rope, stopping the oscillation of the hoisting rope requires that the trolley be accelerated or decelerated at a very slow rate when the trolley is controlled from a remote place or the trolley operates automatically, which reduces the transportation ability of the crane remarkably.